1. Field of the Invention
The present invention relates to an amplifying circuit, such as an RF amplifier for a TV tuner, and more particularly, to an amplifying circuit having a bias voltage setting mechanism.
2. Description of the Related Art
FIG. 4 is a circuit diagram illustrating an AGC circuit which is an example of a conventional amplifying circuit.
An AGC circuit 1 shown in FIG. 4 is an amplifying circuit having a gain setting function, which is provided between an antenna tuning circuit 2 and an RF double-tuned circuit 3.
The AGC circuit 1 is a package 1A that is formed by portions surrounded by one-dotted chain line. Within the package 1A, a first amplifying element 4, a second amplifying element 5, a first resistor r1, and a second resistor r2, which are integrally manufactured during a process of manufacturing a semiconductor, are provided.
Each of the first and second amplifying elements 4 and 5 includes a two-gate type MOSFET. An output from the antenna tuning circuit 2 is input as an input signal Vin to a first gate terminal g1 of the first amplifying element 4, and an AGC (automatic gain control) voltage is input to a second gate terminal g2. The second amplifying element 5 is an FET that controls a bias voltage, and a bias voltage Vb that is controlled by the second amplifying element 5 according to a level of the AGC voltage is applied to the first gate terminal g1 of the first amplifying element 4.
The AGC circuit 1 controls an amplification degree according to the intensity of the input signal Vin, that is, a receiving state of an electric wave signal (intensity of an electric field level). Specifically, the AGC circuit 1 decreases the amplification degree when the electric wave signal is strong and increases the electric wave signal when the electric wave signal is weak.
The AGC circuit 1 is disclosed in JP-A-2001-156565, for example.
The AGC circuit 1 is included in a TV tuner, for example. In this case, it may be required that the bias voltage Vb be changed according to the product specifications of a TV tuner. In the AGC circuit 1, the bias voltage Vb is mainly set by a resistance division ratio of the first resistor r1 and the second resistor r2 each having a fixed resistance.
Therefore, in order to externally change the bias voltage Vb, the resistance division ratio of the first resistor r1 and the second resistor r2 needs to be changed. Specifically, it is possible to consider a first method, in which a third resistor r3 is externally provided in parallel to the first resistor r1 outside the package 1A shown in FIG. 4, for example, between an output terminal of the antenna tuning circuit 2 and a ground GND, so as to lower a combined resistance value at the first resistor r1 side, and a second method, in which a fourth resistor r4 is externally provided in parallel to the second resistor r2 outside the package 1A shown in FIG. 4, for example, between the antenna tuning circuit 2 and the RF double-tuned circuit 3 so as to lower a combined resistance value at the second resistor r2 side.
However, when the first and second methods are used, following problems occur.
In the first method, that is, when the third resistor r3 is externally provided in parallel to the first resistor r1, a capacitive component C due to the third resistor r3 or an internal circuit pattern is connected between the output of the antenna tuning circuit 2 and the ground in an equivalent manner, as shown in FIG. 4. Here, since the capacitive component C serves to reduce the variation range of a tuning frequency at the antenna tuning circuit 2, there occurs a problem in that the capacitive component C has an adverse effect on the antenna tuning circuit 2.
Further, even in the second method, that is, even when the fourth resistor r4 is externally provided in parallel to the second resistor r2, since a capacitive component due to the fourth resistor r4 serves to reduce the variation range of the tuning frequency, the capacitive component C also has an adverse effect on the antenna tuning circuit 2 as described above. Furthermore, since input and output terminals of the AGC circuit 1 are connected through the fourth resistor r4 interposed therebetween, a feedback circuit is formed between the input and output terminals of the AGC circuit 1. For this reason, various problems, such as a variation of a wave form or an abnormal oscillation due to the AGC voltage variation according to an increase of a feedback capacitance, occur.